Examples of known positive electrode active materials used for lithium-ion secondary batteries include composite oxides of lithium and a transition metal, such as LiCoO2, LiNiO2, LiMn2O4, and LiMnO2.
In Published PCT Application 04/082046 (WO 2004/082046), it has been proposed that the compression failure strength should be increased for a lithium-nickel-cobalt-manganese composite oxide powder for positive electrodes. According to the publication, a lithium-nickel-cobalt-manganese composite oxide for positive electrodes of lithium secondary batteries can be obtained that has a high volumetric capacity density and sufficiently satisfies the requirements for safety, cycle performance, and high-current discharge capability.
In Published PCT Application 05/020354 (WO 2005/020354), it has been proposed to use both a first composite oxide powder having a compression failure strength and a second composite oxide powder having a compression failure strength at a certain proportion for a lithium-nickel-cobalt-manganese composite oxide powder for positive electrodes for lithium secondary batteries having a certain composition, to synergistically form a positive electrode with a high filling density. According to the publication, it is described that a positive electrode having a high volumetric capacity density can be thereby obtained synergistically, and such a high volumetric capacity density of the positive electrode can be accomplished without spoiling other characteristics that are necessary for the positive electrode, such as volumetric capacity density, safety, cycle performance, and high-current discharge capability.
JP 2008-266136 A proposes a lithium-nickel-cobalt-manganese-containing composite oxide prepared in the following manner. An aqueous solution of a nickel-cobalt-manganese salt, an aqueous solution of an alkali metal hydroxide, and an ammonium ion supplying substance are supplied consecutively or intermittently to a reaction system, the temperature of the reaction system is set at an almost constant temperature within the range of 30° C. to 70° C., and they are caused to react with each other in a condition in which the pH is kept almost at a constant value within the range of from 10 to 13. Nickel-cobalt-manganese composite hydroxide aggregate particles are synthesized, in which primary particles obtained by depositing nickel-cobalt-manganese composite hydroxide are aggregated to form secondary particles. Further, an oxidizing agent is reacted with the just-mentioned composite hydroxide aggregate particles to synthesize nickel-cobalt-manganese composite oxyhydroxide aggregate particles. Then, at least the just-mentioned oxyhydroxide and a lithium salt are dry-blended and baked to obtain the lithium-nickel-cobalt-manganese-containing composite oxide.